Many processes are known for the preparation of polyurethanes which contain carboxylate groups. For example, these polyurethanes can be prepared by reacting the usual prepolymers which have isocyanate end groups in an organic solvent with aqueous solutions of amino carboxylic acids or their salts to form the corresponding polyurethane ureas containing carboxylate groups as described, for example, in German Auslegeschrift No. 1,495,745, British Pat. No. 1,076,688 and U.S. Pat. No. 3,539,483.
According to another process, dimethylol propionic acid may be used as chain lengthening agent for the synthesis of polyurethanes. The free carboxylic group is then for the most part preserved and can subsequently be neutralized as described for example in U.S. Pat. No. 3,412,055 and German Offenlegungsschrift No. 1,913,271.
According to another known process, polyurethanes which contain free primary or secondary amino groups are reacted with .beta.-propiolactone or the anhydride of a dicarboxylic acid. In this process, the polyurethane is modified with free carboxyl groups as described, for example, in German Auslegeschrift No. 1,237,306.
The use of polyethers or polyesters which have hydroxyl end groups and sulphonate or carboxylate groups in side chains for the synthesis of anionic polyurethane dispersions has also been described, for example in German Auslegeschrift No. 1,570,615. The use of diamines which contain sulphonate or carboxylate groups has been considered for the preparation of polyurethanes which have anionic groups in side chains, for example in German Auslegeschrift No. 1,570,615.
The disadvantages of the known processes is that they can either only be carried out in the presence of organic solvents or require the use of specially synthesized polyesters. Although dimethylol propionic acid can be built into isocyanate prepolymers without the use of solvents, the problem arises in many cases of how to dissolve dimethylol propionic acid in the prepolymer at the required low reaction temperatures. Another disadvantage is that dimethylol propionic acid can only be built in by an isocyanate reaction, with the result that the modified isocyanate prepolymer has a relatively high viscosity.
Esters which contain both hydroxyl groups and free carboxyl groups are already known from the chemistry of dispersible ester resins as described for example in German Offenlegungsschrift No. 2,323,546, U.S. Pat. No. 4,029,617, Belgian Pat. No. 803,346 and U.S. Pat. No. 3,876,582, but the reactive groups are statistically distributed in such products so that these known products cannot be used for synthesizing structurally specific polyurethanes and particularly for synthesizing polyurethanes which are predominantly linear in structure.